Apparatus for monitoring and replenishing toner concentrations

ABSTRACT

A method and apparatus for measuring or measuring and replacing the amount of toner consumed in an electrophotographic reproduction machine wherein the method involves measuring an electrical value corresponding to the charge transferred in a copying operation by the toner to the photoconductor. The electrical value measured is the voltage at a high-resistance resistor through which current flows to the ground from an element charged with a charge corresponding to the transferred charge or the voltage at a capacitor charged from an element that is itself charged with a charge corresponding to the transferred charge or the charge flow from or to the conductive carrier material of the photoconductor. The corresponding charge is the residual charge of carrier material from which toner has been withdrawn by the photoconductor. In response to the measurement an amount of toner is dispensed to the machine which may be intermittent after and in accordance with integration of the charge transferred during the interval between dispensing operations. The apparatus includes an insulated electrically conductive element to receive a charge of the same magnitude as the charge passed to the photoconductor and a measuring instrument which is connected to a toner dispensing device dependent for delivery of toner on the measurement. The conductive element is composed of one or more well insulated deflector sheets fitted in a cascade developing unit wherein the sheets are positioned below the developing zone of the unit so that the spent developer trickles over the sheets beyond the developing zone. The apparatus may also include two toner dispensing devices one of which is dependent upon the number of copies and the other is dependent upon the charge on a capacitor.

United States Patent [1 1 Bock et al.

l APPARATUS FOR MONITORING AND REPLENISI-IING TONER CONCENTRATIONS [75] Inventors: Hartmut Bock, Fischbach;

Helmut Stieger, Wiesbaden; Helmut Jahn, Frankfurt, Hausen; Detlef Winkelmann, Wiesbaden-Biebrich, all of Germany [73] Assignee: I-Ioechst Aktiengesellschaft,

Wiesbaden, Germany [22] Filed: Sept. 25, 1972 [21] Appl. No.1 292,254

[30] Foreign Application Priority Data Primary Examiner-Robert B. Reeves Assistant Examiner.loseph J. Rolla Attorney, Agent, or FirmStevens, Davis Miller & Mosher 1 Oct. 7, 1975 [57] ABSTRACT A method and apparatus for measuring or measuring and replacing the amount of toner consumed in an electrophotographic reproduction machine wherein the method involves measuring an electrical value corresponding to the charge transferred in a copying operation by the toner to the photoconductor. The electrical value measured is the voltage at a highresistance resistor through which current flows to the ground from an element charged with a charge corresponding to the transferred charge or the voltage at a capacitor charged from an element that is itself charged with a charge corresponding to the transferred charge or the charge flow from or to the conductive carrier material of the photoconductor. The corresponding charge is the residual charge of carrier material from which toner has been withdrawn by the photoconductor. In response to the measurement an amount of toner is dispensed to the machine which may be intermittent after and in accordance with integration of the charge transferredduring the interval between dispensing operations. The apparatus includes an insulated electrically conductive element to receive a charge of the same magnitude as the charge passed to the photoconductor and a measuring instrument which is connected to a toner dispensing device dependent for delivery of toner on the measurement. The conductive element is composed of one or more well insulated deflector sheets fitted in a cascade developing unit wherein the sheets are positioned below the developing zone of the unit so that the spent developer trickles over the sheets beyond the developing zone. The apparatus may also include two toner dispensing devices one of which is dependent upon the number of copies and the other is dependent upon the charge on a capacitor. 1

6 Claims, 5 Drawing Figures U.S. Patent Oct. 7,1975 Sheet 1 0f 4 3,910,459

FIG.1

U.S. Patent Oct. 7,1975 Sheet 2 Of4 3,910,459

TONER CONSUMPTION U.S. Patent Oct. 7,1975

Sheet 3 of 4 NUMBER OF COPIES FIG.4

US. Patent Oct. 7,1975 Sheet4 on 3,910,459

APPARATUS FOR MONITORING AND REPLENISI-IING TONER CONCENTRATIONS This invention relates to a method of and apparatus for measuring or measuring and replacing toner consumed in an electrophotographic reproduction machine.

ln electrophotographic reproduction machines, a certain quantity of toner or of other marking material is continuously withdrawn in the copies produced. If fresh toner is not added, this consumption leads to impoverishment in toner and thus to poor, faint copies. This effect is extremely undesirable especially in the case of high-speed electrophotographic reproduction machines.

Various methods have been proposed for dispensing toner. In one method of this kind, an optical reflection of the developed charged image is used for controlling the supply of toner. This method can only be put to practical use if in addition to producing the reflection of the developed charge pattern, the associated original is also scanned. This renders the process complicated, imprecise and likely to break down.

Since the quantity of toner consumed depends upon the charging of the photoconductor as well as determining the blackness of the original, it has been proposed, for the purpose of controlling the supply of toner, to measure the charge of the imagewise exposed photoconductor with the aid of an oscillating capacitor and to utilize the measured value for controlling the supply of toner. However, this method also does not provide direct information on the quantity of toner consumed, said quantity being dependent to a marked degree upon the varying characteristics of the developing unit of the electrophotographic reproduction machine.

The two aforementioned methods are described in US. Pat. No. 2,956,487.

The present invention provides a method of measuring or measuring and replacing toner consumed in an electrophotographic reproduction machine, in which method a measurement is made of an electrical value corresponding to the charge transferred in a copying operation by the toner to the photoconductor, and, in any replacement of toner, an amount of toner is dispensed to the machine corresponding to the said measurement.

In the method of the present invention toner can be dispensed rapidly, accurately, independently of the type of the originals and, in a given machine, largely independently of the machine conditions and the ambient conditions.

The invention is based on the observation that the quantity of toner consumed is always proportional to the charge transferred to the photoconductor by the toner, at constant toner concentration. According to the invention, the charge is measured in a particularly simple manner by measuring an electrical value. Because of the above-mentioned proportionality, the electric current concerned is also proportional to the toner current, i.e., to the amount of toner consumed per unit of time.

The electrical charge transferred to the photoconductor by the toner is always exactly the same as but of opposite sign to the charge that remains in the developing unit, i.e., that which is returned to this unit by the carrier. According to the invention, the toner consumption is measured in a particularly simple manner by measuring the charge transported by the carrier.

The invention further provides, in an electrophotographic reproduction machine, an apparatus for use in the above method. The apparatus includes an insulated electrically conductive element so arranged that in the development step of a reproduction operation of the machine there flows to this element a charge that is of the same magnitude as the charge passed to the photoconductor of the machine by the toner taking part in the development. The apparatus further includes a measuring instrument through which said element can be connected to earth so that when said apparatus is to be used for replacement of toner, said instrument is connected to a device for delivering toner in dependence on the measurement made by said instrument.

A suitable apparatus is one in which the developing unit of an electrophotographic reproduction machine, for example a cascade system, is well insulated within the machine, and the developing unit is connected by way of a high-resistance resistor to earth or to the voltage source of a developing electrode, a magnetic brush or a similar device. The voltage occurring at the highresistance resistor is again proportional to the current from the toner withdrawn by the photoconductor to the amount of toner consumed per unit of time.

In the case of an electrophotographic office copying machine it has been found that the current transferred by the toner to the photographic conductor is largely independent of the toner concentration. One explanation for this phenomenon is that in the case of a relatively high concentration of toner, the carrier particles are covered by more toner than in the case of a lower concentration. The individual toner particles do not therefore become so heavily charged. During development more toner particles are thus passed to the photoconductor but each of these is less heavily charged. In all, however, and in relation to a certain area on the photoconductor that is to be developed, the same charge is transferred as in the case of a lower toner concentration. Thus, if the toner concentration is too great, although more toner per copy is consumed than corresponds to the optimum value, a correspondingly heavier electric current does not flow. Only the amount of toner corresponding to the actual electric current is dispensed by the toner replacement means. In this way the toner concentration gradually decreases towards the optimum amount. A corresponding situation arises in the case of too low a toner concentration. In this instance the same current again flows but on account of the lower toner concentration only a correspondingly small amount of toner is consumed, but more toner, corresponding to the electric current, is dispensed, so that the toner concentration in this case gradually rises towards the optimum amount.

The invention is illustrated by way of example in the accompanying drawings, in which:

FIG. 1 is a diagrammatic section of an electrophotographic reproduction machine comprising a device for measuring the amount of toner taken out in use and for replacing toner;

FIG. 2 is a circuit diagram of the charging circuit used in the method of the invention;

FIG. 3 is a view similar to FIG. 1 of a second embodiment;

FIG. 4 is a graph explaining the mode of operation of the apparatus of FIG. 3; and

FIG. is a diagrammatic illustration of a further device for dispensing toner.

Referring to the drawings, a known type of electrophotographic reproduction machine comprises (FIG. 1) a drum 1 with a photoconductor 2 placed thereon. The photoconductor 2 is uniformly charged by means of a corona discharge device 3 and is exposed in an exposure zone 4 to produce an image. The electrostatic latent charge image resulting from the exposure is developed in a cascade developing zone 5. The developed image is transferred in a transfer zone 6 from the photoconductor 2 to a carrier material 7. When ZnO paper is used, the transfer stage is of course omitted.

According to the invention, the amount of toner withdrawn by the carrier material 7 in the electrophotographic reproduction machine is measured in the following way: the cascade developing means primarily consists of a container 12 (sump 8) and a bucket conveyor system 9, which continually lifts carrier and toner material from the sump 8 and pours it over the imagewise exposed photoconductor 2. Charged toner is deposited on the imagewise charged photoconductor, whereas opposite charges to those of the deposited toner are imparted to the carrier material which falls back into the sump.

In accordance with the invention, the developing zone in the electrophotographic reproduction machine is strictly insulated. This is indicated in FIG. 1 by a dash-and-dot boundary line. The electrical charge transferred by the toner 10 (FIG. 2) to the photoconductor 2 is measured by measuring the electrical charge which is returned by the carrier 11 to the sump and which is equal and opposite to the toner charge. For this purpose, the developing unit or the developer container 12 is connected to earth by way of a highresistance resistor 13. The carrier charge flows through this to earth. Since the electrical charge transferred by the toner 10 to the photoconductor 2 is proportional to the mass of toner adhering to the photoconductor, the electrical voltage occurring at the resistor 13 is proportional to the amount of toner that reaches the photoconductor 2 and adheres to it. Replacement toner can therefore be dispensed in proportion to the voltage occurring at the resistor 13. The voltage is amplified with the aid of an instrument l4, and this instrument 14, in accordance with the voltage occurring at the resistor 13, controls the supply of toner 10 through, for example, a roller 17 containing a groove 16 and fitted in the base of a hopper 15.

The charging circuit will now be described by reference to FIG. 2.

This figure shows diagrammatically at (a) how a carrier particle I] together with a toner particle 10 drops down on to the imagewise exposed photoconductor 2. The toner 10 becomes separated from the carrier particle II in known manner and remains suspended from an oppositely charged point of the photoconductor. It may be assumed that the charging flux occurs in the following manner. The charge of the carrier particle with the toner adhering thereto is equal in amount to the charge of the toner deposited on the photoconductor and after deposition of the toner then flows through the container 12 and the resistor 13 to earth. This charge flow corresponds to a flow of operative charges which travel from earth to the photoconductor drum during the developing operation. The charging circuit is shown in FIG. 2. The signs of the charge illustrated in FIG. 2

may of course be reversed. It is also shown that the charge transferred by the toner to the photoconductor is equal and opposite to the charge carried by the carrier to the container. This latter charge can be measured in a particularly simple manner since the developing unit can be completely separated electrically from the rest of the machine without any great difficulty.

It is however possible instead for the electrically conductive carrier I for the photoconductive material to be insulated and to be connected through a highresistance resistor to earth, and for the voltage occur ring at this resistor to be measured. In this case however, it has to be borne in mind that the resultant voltage can be achieved not only by the transfer of charge effected by the toner, but also by the charge transfer associated for example with the corona device, a cleaning unit or a discharge unit, if special precautions are not taken. In order to measure the only charge transfer that is of importance here, i.e., that achieved by the travel of the toner to the photoconductor, it is necessary in this case to compensate the other flow components or to eliminate them at the moment that development takes place.

In FIG. 3, the positively charged carrier 11 flows back into the sump together with the unused developer. The carrier deposits its charge on a capacitor 20 connected to the container. In contrast to the arrangement illustrated in FIG. 1, measurement and dispensing of fresh toner are achieved not by measuring the current but by measuring the charge. The electrical voltage that builds up at the capacitor 20 is proportional to the charge transmitted by the carrier. Thus the voltage at the capacitor 20 is proportional to the quantity of toner received by the photoconductor 2.

In this arrangement, a timing device 21 of the electrophotographic reproduction machine controls the toner feed, in dependence upon the number of copies, so that a constant quantity of toner is supplied by way of the roller 17 with groove 16. The device can be so designed, for example, that the roller 17 can execute one revolution, controlled by the device 21, for every ten copies. Apart from controlling the roller 17, the device 21 also actuates a switch 22 whereby, while the roller 17 is rotating, the difference between an input voltage V and the voltage at the capacitor 20 is measured on an electrical instrument 23 and is converted into a signal for controlling a dispensing device 24 for fresh toner. The voltage V is such that the condition is satisfied, where C indicates the capacitance of the capacitor 20, O that charge through the capacitor that results if precisely that quantity T of toner is supplied by the roller 17 that is also withdrawn by the photoconductor 2 for the corresponding number of copies. K is the proportionality constant which relates the toner current to the charge current.

In FIG. 4 the number of copies N is shown along the .\'-axis. Successive symbols N1 indicate the numbers of copies according to which the roller 17 dispenses a constant quantity of toner. The stepped curve 30 there fore represents the total quantity of toner supplied through the dispensing device 17 in dependence upon the total number of copies. The curve 31 on the other hand, represents the toner consumption, likewise in dependence upon the number of copies. The curves 32 represent the toner consumption occurring between consecutive deliveries of toner by the roller 17 and are congruent with the corresponding portions of the curve 31. Distances D each indicate how much more toner was consumed between two consecutive deliveries by the roller 17 than was supplied by the said roller. Taking into account the proportionality constant K C,this value D is equal to the voltage U applied to the instrument 23 at the moment when the switch 22 is switched on. By means of the instrument 23 additional toner, corresponding to the magnitude of the voltage impulse U, is dispensed. This dispensing of toner can be achieved with the aid of the separate dispensing device 24 which is substantially similar to the main dispensing device l7. Toner could, however, be dispensed by the instrument 23 by causing the roller 17 to carry out additional rotation.

A discharge resistor 25 and, optionally, a measuring instrument 26 are so rated that the capacitor is completely discharged during the period in which the switch 22 is closed.

In FIG. 5, the cascade developing means which is fully insulated within the electrophotographic reproduction machine is connected to earth through the developer container 12 and a high resistance resistor 13. The voltage occurring at the resistor 13, which is proportional to the electrical charge transmitted by the toner to the photoconductor in a given unit of time, is amplified with the aid of an amplifier 36 and applied to a DC. motor 37. The DC. motor 37 drives a cam disc 39 through a reduction gear 38. During one revolution of the cam disc, the latter actuates a switch 40. This switch 40 starts a timing element 41 which is so designed that a monostable relay 42 is connected to a voltage source 43 for a specific time t. Through a switch 46, the relay 42 connects a DC. motor 44 to a voltage source 45 during this period t. The DC. motor 44 then drives a toner-dispensing device 47 which, for example, and in FIG. 1, may consist of a roller 17 containing a groove 16 and fitted in the base of a tonersupply hopper 15.

It is of course possible to drive the toner dispensing means directly from the DC motor 37. This method however, suffers from the following disadvantage: the DC. motor 37, the speed of which is proportional to the voltage occurring at the resistor 13 and amplified by the amplifier 36, has a torque dependent upon speed. This can result in falsification of the toner being dispensed, particularly in the case of small electrical toner currents. In the arrangement illustrated in FIG. 5, however, the cam disc 39 integrates the revolutions of the motor 37 and toner is dispensed intermittently with the aid of the timing member 41, the DC. motor 44 always being connected to a constant voltage, i.e., the toner dispensing device 47 is always driven with the same torque. One revolution of the cam disc 39 corresponds to a specific level Q of toner charge, and the time t, the motor 44 and the toner dispensing device 47 are such that the quantity T of toner supplied corresponds to this charge level Q.

The method and apparatus of the invention are suitable not only for dispensing toner but with the aid of this direct current-measurement, it is also possible to determine and establish the optimum operating conditions for an electrophotographic reproduction machine. This applies in any case where the variable operating conditions are so selected that no background" occurs in the images. The electrical toner current can,

for example, be passed through an amplifier to a magic 5 eye, with the aid of which the optimum setting of the variable operating conditions can be achieved. This is of particular advantage especially in the initial adjustment and servicing of equipment.-

What is claimed is:

l. A toner dispensing and control system for use in an electrophotographic reproduction apparatus having a photoconductor, a container for the toner, a cascade developer means consisting of a developer zone, a developer container, and a bucket conveyor system for replenishing the developer zone with toner, first means for dispensing toner from the container into said cascade developer means, an actuator device associated with the said first dispensing means for controlling the introduction of the toner into said cascade developer means, means for sensing the electrical charging rate of the material in said cascade developing means comprising the said cascade developer means arranged as a whole as an insulated electrically conductive element to receive a steady stream of carrier material, which is charged opposite to the toner deposited on said photoconductor, image-wise charged and overwhich the developer is cascaded, said sensing means including measuring means and means to be charged in accordance with the charge flow of the carrier material received by said cascade developing means, said means to be charged having one end connected to the developer container and the other end connected to ground, said actuator device associated with said first dispensing means being actuated, in accordance with the flow of a charge current due the applied voltage across the means connected to said developer container and to ground, when the amount of the said charge current during the developing operation falls outside of the range of two limiting values.

2. A toner dispensing and control system according to claim 1, wherein said means to be charged includes a capacitor, the voltage at said capacitor being proportional to the charge transmitted by the carrier material received by the developer container.

3. A toner dispensing and control system according to claim 1, wherein said means to be charged includes a high-resistance resistor, the electrical voltage occurring at said resistor being in accordance with the charge flow of the developer material depleted of toner received by the developer container after cascading over the photoconductor and proportional to the charge of the toner adhering to the image-wise charged photoconductor.

4. A toner dispensing and control system according to claim 1, in which the cascade developing means is fully insulated within the electrophotographic reproduction machine and is connected through a highresistance resistor to ground, an amplifier is connected to the resistor for amplifying the voltage occurring at said resistor, which is proportional to the electric charge transmitted by the developer material depleted of toner, after cascading over the photoconductor, to the developer container, the output of the amplifier is connected to a first electric motor which drives a cam disc connected to the drive shaft of the first electric motor through a reduction gear, the said cam disc is engaged with a switch once for each revolution to close said switch, said switch in its closed position actuates a timing element to connect, through a relay, a second electric motor to a voltage source during a specific constant length of time, and said second electric motor is arranged to drive a toner-dispensing device.

5. A toner dispensing and control system for use in electrophotographic reproduction apparatus having a photoconductor, a first and second container for toner, a cascade developer means consisting of a developer zone, a developer container, and a bucket conveyor system for replenishing the developer zone with toner, first means for dispensing toner from the first container into said cascade developer means, an actuator device associated with the said first dispensing means for controlling the introduction of the toner into said cascade developer means, means for sensing the electrical charging rate of the material in said cascade developer means comprising the said cascade developer means arranged as a whole as an insulated electrically conductive element to receive a steady stream of carrier material, which is charged opposite to the toner deposited on said photoconductor, image-wise charged and over which the developer is cascaded, said sensing means including measuring means and a capacitor to be charged in accordance with the charge flow of the carrier material received by said cascade developer means, said capacitor having one end connected to the developer container and the other end grounded, said actuator device associated with said first dispensing means being actuated, in accordance with the flow of a charge current due to the applied voltage across the capacitor, when the amount of the said charge current during the developing operation falls outside of the range of two limiting values, a second dispensing means for dispensing toner from the second container into said cascade developer means comprising a timing device controlling the actuation of the second dispensing means in accordance with a predetermined number of copies, and a switch device actuated by said timing device to connect an input reference voltage and the voltage at said capacitor to an electrical instrument for measuring the difference between both voltages and to convert the difference voltage into a signal for controlling said first dispensing device.

6. A toner dispensing and control system according to claim 5, including a discharge resistor and a measuring instrument which are connected in parallel with said capacitor and are rated to completely discharge said capacitor during the period in which said switch is closed. 

1. A TONER DISPENSING AND CONTROL SYSTEM FOR USE IN AN ELECTROPHOTOGRAPHIC REPRODUCTION APPARATUS HAVING A PHOTOCONDUCTOR, A CONTAINER FOR THE TONER, A CASCADE DEVELOPER MEANS CONSISTING OF A DEVELOPER ZONE, A DEVELOPER CONTAINER, AND A BUCKET CONVEYOR SYSTEM FOR REPLENISHING THE DEVELOPER ZONE WITH TONER, FIRST MEANS FOR DISPENSING TONER FROM THE CONTAINER INTO SAID CASCADE DEVELOPER MEANS, AN ACTUATOR DEVICE ASSOCIATED WITH THE SAID FIRST DISPENSING MEANS FOR CONTROLLING THE INTRDUCTION OF THE TONER INTO SAID CASCADE DEVELOPER MEANS, MEANS FOR SENSING THE ELECTRICAL CHARGING RATE OF THE MATERIAL IN SAID CASCADE DEVELOPING MEANS COMPRISING THE SAID CASCADE DEVELOPER MEANS ARRANGED AS A WHOLE AS AN INSULATED ELECTRICALLY CONDUCTIVE ELEMENT TO RECEIVE A STEADY STREAM OF CARRIER MATERIAL, WHICH IS CHARGED OPPOSITE TO THE TONER DEPOSITEED ON SAID PHOTOCONDUCTOR, IMAGE-WISE CHARGED AND OVER WHICH THE DEVELOPER IS CASCADED, SAID SENSING MEANS INCLUDING MEASURING MEANS AND MEANS TO BE CHARGED IN ACCORDANCE WITH THE CHARGE FLOW OF THE CARRIER MATERIAL RECEIVED BY SAID CASCADE DEVELOPING MEANS, SAID MEANS TO BE CHARGED HAVING ONE END CONNECTED TO BE DEVELOPER CONTAINER AND THE OTHER END CONNECTED TO GROUND, SAID ACTUATOR DEVICE ASSOCIATED WITH SAID FIRST DISPENSING MEANS BEING ACTUATED, IN ACCORDANCE WITH THE FLOW OF A CHARGE CUR-
 2. A toner dispensing and control system according to claim 1, wherein said means to be charged includes a capacitor, the voltage at said capacitor being proportional to the charge transmitted by the carrier material received by the developer container.
 3. A toner dispensing and control system according to claim 1, wherein said means to be charged includes a high-resistance resistor, the electrical voltage occurring at said resistor being in accordance with the charge flow of the developer material depleted of toner received by the developer container after cascading over the photoconductor and proportional to the charge of the toner adhering to the image-wise charged photoconductor.
 4. A toner dispensing and control system according to claim 1, in which the cascade developing means is fully insulated within the electrophotographic reproduction machine and is connected through a high-resistance resistor to ground, an amplifier is connected to the resistor for amplifying the voltage occurring at said resistor, which is proportional to the electric charge transmitted by the developer material depleted of toner, after cascading over the photoconductor, to the developer container, the output of the amplifier is connected to a first electric motor which drives a cam disc connected to the drive shaft of the first electric motor through a reduction gear, the said cam disc is engaged with a switch once for each revolution to close said switch, said switch in its closed position actuates a timing element to connect, through a relay, a second electric motor to a voltage source during a specific constant length of time, and said second electric motor is arranged to drive a toner-dispensing device.
 5. A toner dispensing and control system for use in electrophotographic reproduction apparatus having a photoconductor, a first and second container for toner, a cascade developer means consisting of a developer zone, a developer container, and a bucket conveyor system for replenishing the developer zone with toner, first means for dispensing toner from the first container into said cascade developer means, an actuator device associated with the said first dispensing means for controlling the introduction of the toner into said cascade developer means, means for sensing the electrical charging Rate of the material in said cascade developer means comprising the said cascade developer means arranged as a whole as an insulated electrically conductive element to receive a steady stream of carrier material, which is charged opposite to the toner deposited on said photoconductor, image-wise charged and over which the developer is cascaded, said sensing means including measuring means and a capacitor to be charged in accordance with the charge flow of the carrier material received by said cascade developer means, said capacitor having one end connected to the developer container and the other end grounded, said actuator device associated with said first dispensing means being actuated, in accordance with the flow of a charge current due to the applied voltage across the capacitor, when the amount of the said charge current during the developing operation falls outside of the range of two limiting values, a second dispensing means for dispensing toner from the second container into said cascade developer means comprising a timing device controlling the actuation of the second dispensing means in accordance with a predetermined number of copies, and a switch device actuated by said timing device to connect an input reference voltage and the voltage at said capacitor to an electrical instrument for measuring the difference between both voltages and to convert the difference voltage into a signal for controlling said first dispensing device.
 6. A toner dispensing and control system according to claim 5, including a discharge resistor and a measuring instrument which are connected in parallel with said capacitor and are rated to completely discharge said capacitor during the period in which said switch is closed. 